The present invention relates to a tuning circuit especially suitable for use in a transmitter-receiver of the phase locked loop frequency-synthesizing type in which a shift of the tuning point corresponding to the intermediate frequency of the receiver unit is achieved interacting with the switching action from reception to transmission.
In a usual transmitter-receiver which uses at least a phase locked loop and which utilizes the controlled output frequency of the voltage-controlled oscillator as the local oscillation frequency for the receiver to produce an intermediate frequency and also as the carrier frequency of the transmitter, it is a well-known practice that, in order to form a system in which a frequency equivalent to the intermediate frequency is made to shift when reception is switched to transmission, phase locked loops are provided separately for transmission and reception in the high-frequency region such as UHF and VHF so that one of the loops may control the other. However, this system inevitably involves a complicated circuit and high cost when put into practical use, though it can obtain accurate frequency phase synchronization. In addition, when the frequency band is increased from 27 MHz to 540 MHz, the requirements that the voltage-controlled oscillator of the phase locked loop must satisfy becomes very severe; and it is practically impossible to obtain such circuit parts as can satisfy the above-mentioned requirements. Moreover, the voltage applied to the variable-capacity element for use in the frequency phase follow-up control of the phase locked loop varies with the channel, and therefore cannot be modulated.